Hello Tazkera

As much I understood, you want to store values of Gx, Gy, Gz in array named varlist. But your syntax
varlist = ['Gx', 'Gy', 'Gz']
will not store values of Gx, Gy, Gz. It stores Gx, Gy, Gz as strings in varlist.
if you want to store values you have to write syntax as
varlist = [Gx, Gy, Gz]

But this will also not work for you. Because if you want to take the slice plot then you have to define varlist as field I think.
I will try to solve it. And we will wait for Nathan reply also.


Regards
Prateek

On Sat, Jun 25, 2016 at 7:24 AM, tazkera haque <h.tazkera@gmail.com> wrote:
Hi Prateek and Nathan,

thanks for your suggestion, I did accordingly, and seems the field is not add up properly. I have attached the error.

At this point I would ask you to kindly take a look at my code too, i am new in python and I want to make sure I am not doing other things wrong along with it.

I am trying to make slice plots of gravitational acceleration through z-axis from the FLASH plotfiles  and make a folder with it.
thanks in advance.

Turhan

On Fri, Jun 24, 2016 at 9:20 AM, Nathan Goldbaum <nathan12343@gmail.com> wrote:


On Fri, Jun 24, 2016 at 6:40 AM, Prateek Gupta <prateekgidolia@gmail.com> wrote:
Hello Turhan,

I am not sure but I think this will work, you can try this...


ds = yt.load("data_file_Name")
grad = ds.add_gradient_fields(("gas","gravitational_potential"))

grad list will have a list of new field names which representing the 3 different components of the field and the magnitude of the gradient , e.g., "gravitational_potential_gradient_x" , "gravitational_potential_gradient_y" , "gravitational_potential_gradient_z" and "gravitational_potential_gradient_magnitude"

Now for getting the negative gradient of gravitational potential we can derive field as follows:


####...x-component of negative of gradient of gravitational potential...
def _GradientX(field, data)
     Gx = -1.0*data["gravitational_potential_gradient_x"]
     return Gx
yt.add_field("Gx", function=_GradientX, take_log=False, units="code length/ code time**2")

####...y-component of negative of gradient of gravitational potential...
def _GradientY(field, data)
     Gy = -1.0*data["gravitational_potential_gradient_y"]
     return Gx
yt.add_field("Gy", function=_GradientY, take_log=False, units="code length/ code time**2")

####...z-component of negative of gradient of gravitational potential...
def _GradientZ(field, data)
     Gz = -1.0*data["gravitational_potential_gradient_z"]
     return Gz
yt.add_field("Gz", function=_GradientZ, take_log=False, units="code length/ code time**2")



Otherwise you can do this also, 
grad = -1.0*ds.add_gradient_fields(("gas","gravitational_potential"))

This isn't quite right. The ds.add_gradient_fields function returns a set of field names, not a field object itself. If you want to add the negative signs to the field definition, you'll need to define some derived fields as Prateek suggested above.
 

I think this will work but this will also make the magnitude negative. But if you want to use only 3-components then this works and you can access it , e.g., data["gravitational_potential_gradient_x"] and similar for y and z component.



Please let me know if this work.



Regards
Prateek Gupta

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